Tripod type constant velocity joint and grease for the joint

ABSTRACT

In order to reduce vibration and improve durability at a low cost through the reduction of induced thrust, such a grease is used that has SRV friction coefficient μ h  under a contact pressure of 2176 MPa in a range from 0.060 to 0.080 and decreasing rate of SRV friction coefficient μ 1  under a contact pressure of 3954 MPa with respect to the friction coefficient μ h , namely (μ h −μ 1 )/μ h ×100%, in a range from 40 to 60%. The outer joint member  1  is filled with this grease.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a tripod type constant velocityjoint used in the power transmission apparatuses of automobiles orvarious industrial machines, and to a grease used in the joint.

[0002] A tripod type constant velocity joint has a tripod member 2 thathas three trunnions 2 a that project in the radial direction and aredisposed at equal intervals along the circumference as shown in FIG. 7.Each of the trunnions 2 a has a roller 3 rotatably mounted thereon via aneedle roller 4, the rollers 3 being housed in three grooves 1 a formedat equal intervals on the inner circumference of an outer joint member1. Each of the rollers 3 is guided by a roller guide surface 1 a 1 ofthe corresponding track groove 1 a, whereby to be capable of movingalong the track groove 1 a while rotating about the trunnion 2 a.

[0003] As the roller 3 rolls over the roller guide surface 1 a 1 whilerotating about the axis of the trunnion 2 a, the outer joint member 1and the tripod member 2 are allowed to undergo relative axialdisplacement with respect to each other, namely relative displacement inthe axial direction (plunging) between two shafts to be coupled. At thesame time, each of the trunnions 2 a is allowed to make displacement inthe axial direction with respect to the roller guide surface 1 a 1 asthe phase of rotation changes when rotation torque is transmitted whilethe outer joint member 1 and the tripod member 2 maintain apredetermined operation angle θ.

[0004] Tripod type constant velocity joints of this type are applied todrive shafts, propeller shafts and the like of front-wheel drivevehicles and 4-wheel drive vehicles, because of less sliding movementand less sliding resistance.

[0005] In the tripod type constant velocity joint described above, theroller 3 and the track groove 1 a are disposed in an obliquely crossingarrangement when rotation torque is transmitted while the outer jointmember 1 and the tripod member 2 maintain the operation angle θ. As aresult, a thrust force is induced in the axial direction when the roller3 moves in the track groove 1 a in the axial direction of the outerjoint member 1. The induced thrust that is characteristic of the tripodtype constant velocity joint may cause mechanical vibration and,particularly in the case of an automobile, may increase noise and lowerNVH (noise vibration harshness) characteristic. Thus various measureshave been proposed for the reduction of induced thrust. The measures forthe reduction of induced thrust include the replacement of the singleroller 3 shown in FIG. 7 with a dual roller arrangement, andmodification of the shape and material of mechanical parts. However,such measures require additional investment on the existing productionfacilities, thus leading to such drawbacks as significant increase inthe manufacturing cost and increased weight. Also as the automobilestend to have increasing output power, joints are required to haveincreasingly higher durability, but measures taken in the prior art toimprove the durability inevitably lead to such disadvantages asincreased cost and weight. Accordingly, an object of the presentinvention is to provide a tripod type constant velocity joint and agrease used in the joint capable of achieving lower vibration throughdecreasing the induced thrust and improving the durability, at a lowercost.

SUMMARY OF THE INVENTION

[0006] The present inventors focused on the condition of lubricationbetween the roller 3 and the roller guide surface 1 a 1 as a factor thatdetermines the durability and induced thrust. Satisfactory lubricationin this interface is expected to make the rolling motion of the roller 3smoother, thus causing favorable effects on the durability and inducedthrust.

[0007] A grease film of lower strength generally results in metalcontact due to breakage of the grease film, having adverse effects onthe generation of induced thrust and durability. In a tripod typeconstant velocity joint, in particular, slippage between the roller 3and the roller guide surface 1 a 1 increases significantly whenoperating with a large operation angle (approximately θ=10° to 25°),thus making the grease film more likely to break in such contact areas.Therefore, a grease used in the tripod type constant velocity joint isdesired to have grease film strength high enough to reliably prevent thegrease film from breaking during operation with a high operation angle.

[0008] When the grease film has high strength, on the other hand,initial compatibility of the grease with the contact surface becomeslower. Lower initial compatibility of the grease causes a delay insmoothing of the rolling surface and concentration of stress in higherportions on the surface, thus giving rise to a possibility of flaking ofthe contact surface or other failures due to the generation andpropagation of cracks, eventually resulting in shorter service life ofthe joint. Therefore, the grease is desired to have a high film strengthwhich is, however, not too high.

[0009] Grease film strength and initial compatibility are of conflictingnature with each other, as described above, and it is difficult tosatisfy the requirements for both properties at the same time.

[0010] Based on the consideration described above, the inventorsconducted friction tests on various kinds of grease in order todetermine optimum grease properties, particularly friction coefficientμ.

[0011] Through this investigation, it has been found that grease filmstrength and initial compatibility can be well balanced and best effectscan be achieved in decreasing the induced thrust and improving thedurability by using a grease that has SRV friction coefficient μ_(h)under a contact pressure of 2176 MPa in a range from 0.060 to 0.080 anddecreasing rate of SRV friction coefficient μ₁ under a pressure of 3954MPa with respect to the friction coefficient μ_(h) in a range from 40 to60%, the decreasing rate being represented by the following formula (1):

(μ_(h)−μ₁)/μ_(h)×100(%)  (1).

[0012] It has also been verified that a grease which shows a wear amountof 10 mg or lower in Fafnir friction oxidation test is effective inreducing the variation of induced thrust and achieving less vibration,even when operating with a large operation angle θ.

[0013] It will be more advantageous, for reducing the induced thrust andimprove the durability, to add 0.3 to 1.5 wt % of molybdenum disulfideto such a grease as described above, or to apply shot peening treatmentto the inner circumference of the outer joint member.

[0014] According to the present invention, as described above, since thefriction coefficient of the grease is optimized, satisfactorylubrication can be maintained between the roller and the roller guidesurface, and reduction of vibration through the reduction of inducedthrust and improvement of durability can be achieved. Moreover, theincrease in the cost and weight can be avoided since it suffices tochange only the grease without need to change the shapes or materials ofother parts.

[0015] With a grease that shows the wear amount of 10 mg or lower inFafnir friction oxidation test, it is made possible to improve thefretting resistance while making a flat profile of induced thrust andreducing the magnitude of the thrust.

[0016] When 0.3 to 1.5 wt % of molybdenum disulfide is added to thegrease, initial compatibility of the grease is improved and thereforethe induced thrust can be decreased and the durability can be improvedfurther. In this case, it may be also possible to eliminate the finishgrinding operation in the manufacturing process of the outer jointmember.

[0017] When shot peening treatment is applied to the inner circumferenceof the outer joint member, further reduction of vibration and elongationof service life can be achieved because of the improved effect ofpreventing fretting and other reason.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the accompanying drawings:

[0019]FIG. 1A is a sectional view of a tripod type constant velocityjoint in the axial direction, and FIG. 1B is a sectional view thereof inthe radial direction:

[0020]FIG. 2 is a side view schematically showing a setup for SRVfriction test:

[0021]FIG. 3 shows a test result obtained with the SRV friction test:

[0022]FIG. 4 shows results of durability and NVH characteristic tests:

[0023]FIG. 5 is an exploded perspective view schematically showing aFafnir friction oxidation tester:

[0024]FIG. 6 shows the results of Fafnir friction oxidation test: and

[0025]FIG. 7 is a sectional view of the tripod type constant velocityjoint in the axial direction disposed with an operation angle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Preferred embodiment of the present invention will now bedescribed below with reference to FIG. 1A through FIG. 7.

[0027] As shown in FIG. 1A and FIG. 1B, the tripod type constantvelocity joint of the present invention comprises an outer joint member1 having a shape of substantially bottomed cylinder, a tripod member 2that has three trunnions 2 a, disposed at equal intervals along thecircumference and projecting in the radial direction, and is inserted inthe outer joint member 1, and ring-shaped rollers 3 mounted rotatably onthe respective trunnions 2 a of the tripod member 2 via rolling elements4 such as needle rollers. Provided on the inner circumference of theouter joint member 1 at equal intervals in the circumferential directionare three track grooves 1 a extending in the axial direction of theouter joint member 1, with the rollers 3 mounted on the respectivetrunnions 2 a being housed in the corresponding track grooves 1 a. Wallsurfaces on both sides of each track groove 1 a serve as roller guidesurfaces 1 a 1 that make fitting contact with outer circumferencesurface 3 a of the roller 3, the roller guide surfaces 1 a 1 extendingin the axial direction of the outer joint member 1. In this embodiment,a case is described where the roller guide surface 1 a 1 has a concavecylindrical surface and the outer circumference surface 3 a of theroller 3 is formed in a convex spherical surface with the center lyingon the axial line of the trunnion 2 a. Coupled with the tripod member 2on the inner circumference thereof by means of serration or the like isa shaft 5.

[0028] The tripod type constant velocity joint is normally made byinserting a tripod assembly, comprising the rolling elements 4 and therollers 3 assembled on the tripod member 2, inside the outer jointmember 1, and filling the inside of the joint (for example, the insideof the outer joint member 1) with a grease. Fitted between the outerjoint member 1 and the shaft 5 is an elastic boot not shown in thedrawing, for the purpose of preventing the grease from leaking andprotection against external impact.

[0029] As described previously, the grease used in the tripod typeconstant velocity joint is desired to have grease film strength highenough to reliably prevent the grease film from breaking duringoperation with a large operation angle. However, a grease havingexcessively high grease film strength cannot be used for the reason ofbalance with the initial compatibility.

[0030] Based on the consideration described above, a friction testdescribed below was conducted to determine the grease properties thatallow it to achieve a good balance between the grease film strength andthe initial compatibility. Based on the assumption that grease filmstrength could be evaluated by the load dependence of frictioncoefficient μ of the grease, grease film strength was evaluated by thedecreasing rate of the friction coefficient as the load was increased atconstant steps in this test. It can be said that a low decreasing rateof friction coefficient indicates low load dependence of frictioncoefficient and hence high grease film strength, and a high decreasingrate of friction coefficient indicates high load dependence and lowgrease film strength.

[0031] The friction test was conducted in conformity with the SRV testprocedure specified in ASTM D5706 and D5707. In the procedure of thistest, as outlined in FIG. 2, a ball 11 is placed on a disk 10 coatedwith one of various greases (13 kinds of grease in this embodiment), avertical load P and horizontal vibration of amplitude A are applied tothe ball 11, while measuring the friction coefficient μ.

[0032] First, at the room temperature, contact pressure between the disk10 and the ball 11 was set to 2176 MPa (load P=100 N). Under thisconditions, vibration of amplitude A=1 mm and frequency 15 Hz wasapplied to the ball 11 and friction coefficient μ_(h) was measured foreach kind of grease. Then the contact pressure was increased to 3954 MPa(load P=600 N) and friction coefficient μ₁ was measured under the sameconditions as the above (room temperature, A=1 mm and frequency 15 Hz).Decreasing rate of μ₁ with respect to the friction coefficient μ_(h) wasdetermined by the following formula (1):

(μ_(h)−μ₁)/μ_(h)×100(%)  (1).

[0033] With μ_(h) plotted along the abscissa and the calculateddecreasing rate plotted along the ordinate, it can be seen that there isa substantially proportional relationship between μ_(h) and thedecreasing rate as shown in FIG. 3. This implies that the higher thefriction coefficient μ_(h) of a grease, the greater the decreasing rateof the friction coefficient and hence the lower the grease filmstrength.

[0034] Then service life test and measurement of induced thrust wereconducted on a sample tripod type constant velocity joint filled witheach kind of grease. The service life test was conducted under theconditions of rotation torque 834 Nm, operation angle θ=6° androtational speed 230 rpm, and the induced thrust was measured under theconditions of rotation torque 294 Nm, operation angle θ=6° androtational speed 150 rpm. Results of both tests are shown in FIG. 4(‘NVH’ in the drawing indicates the result of induced thrustmeasurement). Regions A through C in FIG. 4 represent the regions of thegraph shown in FIG. 3 divided by lines of μ_(h)=0.060 and 0.08 and thelines of decreasing rates of 40% and 60%.

[0035] As can be seen from FIG. 4, greases belonging to region B showsatisfactory performance in both the service life and NVHcharacteristic. Greases belonging to region A, on the other hand, hassatisfactory NVH characteristic but not in durability, while greasesbelonging to region C show poor performance in both durability and NVHcharacteristic. This may be because a grease belonging to region A showsgood NVH characteristic due to high grease film strength but shows lowdurability due to poor initial compatibility, while a grease belongingto region C shows good initial compatibility but the low grease filmstrength makes the film more likely to break, resulting in a conditionof lubrication so poor that cannot be covered by the good initialcompatibility. Based on the forgoing discussion, a grease used in thetripod type constant velocity joint is preferably one that belongs toregion B, namely that has SRV friction coefficient μ_(h) under a contactpressure of 2176 MPa (load 100 N) in a range from 0.060 to 0.080inclusive and decreasing rate (formula (1)) of SRV friction coefficientμ₁ under a contact pressure of 3954 MPa with respect to the frictioncoefficient μ_(h) in a range from 40 to 60%.

[0036] While there is sliding friction working between the roller 3 andthe roller guide surface 1 a 1 as mentioned previously, variation in theinduced thrust is expected to increase as the amount of fretting wear inthe contact surface increases. Accordingly, fretting resistance wasevaluated on various greases by using a Fafnir friction oxidationtester.

[0037] The Fafnir friction oxidation test was conducted on such anarrangement as shown in FIG. 5 where roller bearings 13, 14 filled with1 gram of a sample grease are fixed on a bearing holder 15 a of avibration stage 15, a shaft 21 is inserted through an upper chuck 12,the bearings 13, 14, the vibration stage 15, a lower chuck 16, a springguide 17, a spring 18, a spacer 19 and a washer 20 with the tip of theshaft 21 being screwed into a bolt 22, and a predetermined load (550 lb)is applied to the bearings 13,14 with the spring 18 by adjusting thetightening of the bolt 22. Under this condition, the vibration stage 15is connected to a motor via a connecting rod not shown. After runningthe motor to oscillate the vibration stage 15 (angular amplitude of 0.21radian at frequency of 30 Hz) for a predetermined period of time,fretting resistance of the sample grease (6 kinds in this embodiment) isevaluated in terms of the total weight loss of the bearings 13, 14.While the procedure of fretting resistance evaluation test specified inASTM employs W-5/8 (made in U.S.A.) for the bearings 13, 14 with thetest being continued for 22 hours, bearing 51204 made in Japan and testperiod of two hours are employed in the test of the present invention.

[0038] Then variation of the induced thrust was measured on a sample ofthe tripod type constant velocity joint filled with each kind of grease.The measuring conditions were rotation torque of 294 Nm, operation angleθ of 6° and rotational speed of 150 rpm. Induced thrust was measured atone-minute intervals during five minutes of operation, and thedifference between maximum and minimum values of the measurements wastaken as the variation of induced thrust. The test results are shown inFIG. 6.

[0039]FIG. 6 shows that the variation of induced thrust increasessignificantly when the wear amount measured in Fafnir friction oxidationtest increases beyond 10 mg. Thus a grease that shows wear not greaterthan 10 mg in Fafnir friction oxidation test should be used. This makesit possible to obtain flat characteristic of induced thrust even under alarge operation angle θ, thus improving the NVH characteristic.

[0040] The outer joint member 1 of the tripod type constant velocityjoint is typically made of a steel such as carbon steel that is formedinto a predetermined configuration by cold forging or the like andsubjected to heat treatment such as induction hardening in order toensure the required levels of strength, durability and wear resistance,followed by grinding of portions that require high precision therebyfinishing the part to the predetermined dimensions and completing theproduct. Recently, studies have been made on the possibility ofeliminating the grinding operation in the final stage of themanufacturing process, for the purpose of reducing the manufacturingcost. When the grinding operation is omitted, it is anticipated that theaccuracy of the track groove 1 a of the outer joint member 1 woulddecrease with the surface roughness increasing, leading to poorercharacteristic of induced thrust and lower durability. When 0.3 to 1.5wt % of molybdenum disulfide is added to the grease described above,initial compatibility of the grease will be improved and therefore NVHcharacteristic and the durability can be improved. Thus it is madepossible to eliminate the grinding operation. It goes without sayingthat MoS₂ may be added also in the case of applying grinding operation,in which case NVH characteristic and the durability can be improvedfurther. The reason for limiting the amount of MoS₂ to be added in therange from 0.3 to 1.5 wt % is that addition of less than 0.3 wt % cannotachieve significant effect of reducing the vibration and, when theamount of addition is more than 1.5 wt %, the effect of reducing thevibration reaches a plateau and, in worst case, MoS₂ powder exertsabrasive action on the rolling surface, eventually leading to increasedwear and shorter service life.

[0041] The inner circumference surface of the outer joint member 1 thathas been induction hardened may also be subjected to shot peeningtreatment. When this is done, as the surface is hardened and residualcompressive stress is generated, fretting can be prevented and furtherreduction of vibration and elongation of service life can be achieved.It is a common practice to apply phosphoric acid coating treatment(Bonderite treatment) to the outer joint member 1 before applying theforging operation with the result of a hard and brittle oxide coat beingformed on the surface as P is concentrated through the subsequentprocess of hardening, whereas applying shot peening removes the oxidecoat and makes it possible to mitigate the wearing of the rollingsurface. Also it is made possible to form microscopic pools for thegrease out of microscopic dimples formed on the surface, thus reducingthe frictional force of the rolling surface.

[0042] The present invention can be applied to greases of variouscompositions regardless of the type of base oil (mineral oil,poly-α-olefin, diester, etc.) and the type of the thickener(lithium-based, urea-based, etc.). Although FIG. 1 shows a case where asingle roller is used, the grease of the present invention may also beused for a dual roller assembly comprising an inner roller and an outerroller with the outer roller adapted to be capable of swinging.

[0043] While there has been described what are at present considered tobe preferred embodiments of the invention, it will be understood thatvarious modifications may be made thereto, and it is intended that theappended claims cover all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:
 1. A grease for a tripod type constant velocityjoint having SRV friction coefficient μ_(h) under a contact pressure of2176 MPa in a range from 0.060 to 0.080 and decreasing rate of SRVfriction coefficient μ₁ under a contact pressure of 3954 MPa withrespect to the friction coefficient μ_(h) in a range from 40 to 60%, thedecreasing rate being represented by the following formula (1):(μ_(h)−μ₁)/μ_(h)×100(%)  (1).
 2. A grease for a tripod type constantvelocity joint according to claim 1, wherein said grease shows a wearamount of 10 mg or lower in Fafnir friction oxidation test.
 3. A greasefor s tripod type constant velocity joint according to claim 1, wherein0.3 to 1.5 wt % of molybdenum disulfide is added thereto.
 4. A tripodtype constant velocity joint comprising an outer joint member havingtrack grooves formed in the axial direction at equal intervals on aninner circumference thereof and having roller guide surfaces extendingin the axial direction on both sides of each track groove, a tripodmember having trunnions projecting in the radial direction and disposedat equal intervals along the circumference, rollers mounted rotatably onthe respective trunnions and a grease filled the inside of the joint,the rollers being guided on the respective roller guide surfaces of thetrack grooves, wherein the grease has SRV friction coefficient μ_(h)under a contact pressure of 2176 MPa in a range from 0.060 to 0.080 anddecreasing rate of SRV friction coefficient μ₁ under a contact pressureof 3954 MPa with respect to the friction coefficient μ_(h) in a rangefrom 40 to 60%, said decreasing rate being represented by the followingformula (1): (μ_(h)−μ₁)/μ_(h)×100(%)  (1).
 5. A tripod type constantvelocity joint according to claim 4, wherein the grease shows a wearamount of 10 mg or lower in Fafnir friction oxidation test.
 6. A tripodtype constant velocity joint according to claim 4, wherein 0.3 to 1.5 wt% of molybdenum disulfide is added to said grease.
 7. A tripod typeconstant velocity joint according to one of claims 4 through 6, whereinthe inner circumference surface of the outer joint member is subjectedto shot peening treatment.